1. Field of the Invention
The invention relates to formulations based on powders which comprise a non-conductive material and activators and if appropriate pigments and charge control agents.
These formulations are used for pretreatment of substrate surfaces, for example surfaces of plastics, with the aim of subsequent metallization in currentless metallization baths.
The metallized surfaces of plastics are employed for the purpose of shielding from electromagnetic waves, as a light barrier, as a structured metal surface for conductor tracks for electrical circuits, as a dot and dash code for data processing, for permanently writing data to CD discs, as electrical switches, for tablet blisters and packagings and other similar purposes.
2. Description of the Related Art
It is known that polymeric materials must be pretreated before chemical and subsequent electrometallization in order to achieve adequate adhesion of the layer of metal to the surface of the material. The pretreatment is mainly carried out by etching the polymer surface with environmentally polluting chromic/sulphuric acids. However, working with chromic/sulphuric acid, SO.sub.3 vapour or other oxidizing agents is accompanied by a deterioration in the physical properties, such as the impact strength and electrical surface resistance, of the polymeric material. Trouble is furthermore often caused by traces of 6-valent chromium, which can rapidly lead to poisoning of the metal baths.
The known processes for currentless metallization of materials furthermore comprise several process stages and have the disadvantage that they cannot be used directly on all polymers. Chemical swelling or physical roughening often additionally has to be carried out.
It has therefore already been proposed to activate the polymer surfaces with organometallic catalysts under very mild conditions (US 3 560 257 and EP-A 81 129). However, this progressive method is also not universally applicable. Moreover, the use of solvents often leads to the start of stress corrosion cracking of the polymeric injection-moulded component under tensile or compressive stress.
Other processes, such as are described in U.S. Pat. Nos. 3,560,257 and 4,017,265 and also DE-A 3 627 256, have the disadvantage that they require relatively large amounts of expensive noble metal activators.
Specific adhesion-promoting coatings of plastic furthermore are known from DE-A 3 814 506. However, these have to be activated in activation baths before their metallization, which can lead to geometry-related foreign deposition in cases of partial metallization.
Finally, DE-A 40 36 591 describes an economical, universally applicable process for chemical metallization in which, without prior etching with oxidizing agents, surfaces of materials based on glasses, metals and, in particular, plastics can be provided with a firmly adhering metal coating deposited by a wet-chemical route. The process comprises coating substrate surfaces with a specific primer based on a polymeric organic film or matrix-forming agent, which also additionally contains an additive, metallization activators, fillers and solvent, and subsequently metallizing the surfaces with a firmly adhering deposit.
The disadvantage of the process mentioned last is that the primer described necessarily contains organic solvents. However, solvent-containing paints are more and more under pressure for environmental reasons. Furthermore, it has proved to be a disadvantage that a relatively dense spray mist is formed when the primer described above is sprayed onto substrate surfaces.
DE-A 41 07 644 furthermore describes a process for chemical metallization in which substrate surfaces are coated with a hydroprimer which comprises an aqueous dispersion of a polymeric film-forming agent, metallization activators and if appropriate filler. After drying, the substrate surfaces coated with this hydroprimer can be provided with metal coatings in metallization baths.
The disadvantage of the process mentioned last is that aqueous formulations are employed, and if water-soluble activators are used, there is the danger that the metallization baths will be contaminated with the activator dissolved out of the coating, and residual water contents have to be removed from the coating. The activator of the coating furthermore must be brought into an active form by preliminary reduction, for example with dimethylaminoborane (DMAB), in an additional step in order to gene, rate uniform layer thicknesses of the metal deposit in the subsequent metallization step.
Finally, DE-A 41 11 817 describes another economical process for chemical metallization in which substrate surfaces likewise are coated with an aqueous dispersion of a specific polyurethane, the metallization activators added being present quantitatively in water-insoluble and active form. This means that contamination of the metallization baths is avoided and uniform metal layer thicknesses are produced on the substrate surface.
One disadvantage of this formulation is that the residual water of the coating must be removed.